A new servo writing technique that magnetically transfers servo data for positioning a magnetic head for writing and reading data to a magnetic recording medium has been attracting much attention. For facilitating the new servo writing technique, the servo data is written in a form of soft magnetic patterns, shaped with lines or islands and embedded in the surface portion of a master disk. The new servo writing technique magnetically transfers the servo data to the surface of the magnetic recording medium by positioning the master disk in tight contact with or in close proximity to the magnetic recording medium and by applying a magnetic field from the outside. Since the new servo writing technique transfers the position data (servo pattern) for controlling the magnetic head of a hard disk drive (HDD) using a magnetic film efficiently in a very short time, the new servo writing technique has been expected to drastically reduce the manufacturing costs of the HDD, to improve the recording density of the HDD, and to create new additional values for the HDD.
FIG. 7(a) is a cross sectional view showing a step of conventional magnetic transfer. FIG. 7(b) is another cross sectional view showing another step of conventional magnetic transfer. Referring now to FIG. 7(a), a magnetic recording medium (magnetic disk) 1 is magnetized uniformly to the right-hand side of the figure (circumference direction of the magnetic recording medium) 6 by an external magnetic field Hex, which is much higher than the coercive force Hc of the magnetic recording medium 1, and applied from a permanent magnet 4 moving in the circumference direction 6 of the magnetic recording medium 1 with a certain spacing kept therebetween (the step of initial magnetization). In FIG. 7(a), a yoke 5 made of a soft magnetic material is shown. The yoke 5 and the permanent magnet 4 constitute a ring head.
Referring now to FIG. 7(b), a master disk 3 is positioned in close proximity to or in close contact with the magnetically initialized magnetic recording medium 1. The master disk 3 includes magnetic layers 3a (Co soft magnetic layers) scattered and embedded in the surface portion 3b of the master disk 3. Magnetic transfer is conducted by applying an external magnetic field Hex to the direction 7 opposite to the direction 6 of the external magnetic field Hex applied for the initial magnetization from the permanent magnet 4 moving above the master disk 3 positioned in close proximity to or in close contact with the magnetic recording medium 1.
The magnetic transfer process will be described below more in detail. The leakage magnetic field (the direction thereof being the same as the direction of the magnetic field for magnetic transfer and opposite to the direction of the magnetic field for the initial magnetization) from the moving permanent magnet 4 reaches the magnetic layer in the surface portion of the magnetic recording medium 1 through the surface portion 3b of the master disk 3. The leakage magnetic field that has reached the surface portion of the magnetic recording medium 1 inverts the initial magnetization, resulting in recorded magnetization with a high coercive force. In the embedded magnetic layers 3a, the leakage magnetic field expands parallel to the surface of the master disk 3, i.e., along the paths with lowest magnetic resistance. Since the leakage magnetic field hardly reaches the magnetic layer in the magnetic recording medium 1 through the embedded magnetic layers 3a, the initial magnetization remains without being inverted in the portions of the magnetic layer of the magnetic recording medium 1 below the embedded magnetic layers 3a. Thus, a negative pattern of the embedded magnetic layers 3a is transferred to the magnetic recording medium 1. The magnetic transfer technique described above does not utilize the leakage magnetic fields from the embedded magnetic layers 3a for magnetizing the magnetic recording medium 1 but utilizes the embedded magnetic layers 3a as a mask for interrupting the leakage magnetic field from the permanent magnet 4. In short, the leakage magnetic field from the permanent magnet 4 selectively magnetizes the magnetic recording medium 1 through the surface portion 3b of the master disk 3. See for example, Japanese Unexamined Laid Open Patent Application 2001-34939.
Now the relation between the strength Hex of the external magnetic field for magnetic transfer and the coercive force Hc of the magnetic recording medium 1 will be described. As described above, fine soft magnetic patterns 3a are embedded in the surface of the master disk 1. FIG. 8(a) shows cross sectional views illustrating the distribution of the magnetic field around the soft magnetic patterns 3a. FIG. 8(b) is a graph illustrating the distributions of the parallel components of the magnetic fluxes on the magnetic recording medium 1.
As illustrated in FIG. 8(a), the magnetic fluxes converge to the soft magnetic patterns 3a in the regions where the soft magnetic patterns 3a are located. The magnetic fluxes, once converged to the soft magnetic patterns 3a, diverge from the soft magnetic patterns 3a into the regions where no soft magnetic pattern 3a is located. The magnetic field Ha beneath the central portion of the soft magnetic pattern 3a is lowest since the magnetic fluxes converge beneath the central portion of the soft magnetic pattern 3a. The magnetic field Hb at both ends of the soft magnetic pattern 3a is the highest since the magnetic fluxes converge to both ends of the soft magnetic pattern 3a. The magnetic field Hg at the center of the region between the adjacent soft magnetic patterns 3a is lower than the magnetic field Hb at both ends of the soft magnetic pattern 3 since the once converged magnetic fluxes diverge at the center of the region between the adjacent soft magnetic patterns 3a. 
FIG. 8(b) plots the relations between the external magnetic field Hex and the above described magnetic fields Ha, Hb, and Hg. Due to the existence of the soft magnetic patterns 3a, the magnetic fields Hb and Hg increase with increasing magnetic field Hex, always keeping higher values than that of the corresponding external magnetic field Hex. Therefore, the curves for the magnetic fields Hb and Hg are always above the broken line (having the gradient of 1) on the graph. In contrast, the magnetic field Ha is always lower than the corresponding external magnetic field Hex due to the existence of the soft magnetic patterns 3a. The curve for the magnetic field Ha is always below the broken line. The reference symbol Ht in FIG. 8(b) designates the magnetic field strength at which the soft magnetic patterns 3a saturate magnetically. Therefore, the magnetic field Ha applied to the magnetic recording medium 1 beneath the soft magnetic patterns 3a increases sharply, when the external magnetic field Hex applied is higher than the magnetic field Ht.
Therefore, the magnetic transfer to the magnetic recording medium 1 with the coercive force Hc can be facilitated by applying an external magnetic field Hex with the following relations to the magnetic recording medium 1: Hb>Hc, Hg>Hc, and Ha<Hc. The conventional magnetic transfer technique described above is effective for the magnetic transfer to a magnetic recording medium having only a single recording layer. It is impossible, however, for the conventional magnetic transfer technique to transfer the soft magnetic patterns embedded in a master disk independently to two recording layers included in a magnetic recording medium.
Accordingly, there is a need to provide a magnetic transfer method that transfers magnetic signals independently to two recording layers included in a magnetic recording medium, along with a magnetic recording medium that has two recording layers that can transfer different magnetic signals independently to the respective recording layers. The present invention addresses this need.